Vigorous research and development efforts are being thus made for integrated circuits composed of field-effect transistors using gallium-arsenide providing an electron mobility which is three to five times higher than that achievable by silicon. As well known in the art, field-effect transistors of gallium-arsenide provide higher cutoff frequencies as they are scaled down. The increased cutoff frequency will do justice to a semiconductor integrated circuit with a relatively small load requirement and a relatively low integration density. It is, however, presumed those skilled in the art that increasing the density of the integrated circuit as achieved by the miniaturization of the component devices might be detrimental to achievement of faster operation because of the low load driving capabilities of the field-effect transistors. There is, for this reason, a demand for transistors providing increased load driving capabilities or, more specifically, for bipolar transistors having improved cutoff frequency characteristics. A prime object of the present invention is to provide a bipolar transistor with an increased cutoff frequency and more adaptability to integration and a process of fabricating such a transistor.